The present invention relates to an apparatus and method for detecting chemical/biological substances which are remote or otherwise inaccessible, such as, for example, the sensing, analysis, location and identification of gases, solid or liquid materials, contaminants and pollutants. More specifically, the present invention relates to a portable mini-lidar apparatus for the remote stand-off sensing of chemical/biological agents and the method for detecting same.
A sensor having the ability for portable, remote, stand-off, high speed and efficient sensing of chemical or biological agents would be highly desirable as there are numerous civilian and military applications which require or could benefit from such a device. Such applications include the detection classification of chemical spills and seepage thereof into the land and waters, and remotely sensing the chemical or biological agents which may be intentionally released on a population.
The release of chemical or biological agents either intentionally or accidentally often requires an emergency response in order to interrogate the contamination and determine the composition and extent of the contamination. This work is typically performed by what is known in the art as xe2x80x9cfirst respondersxe2x80x9d such as police or fire units which are charged with assessing the nature of the contamination in order to determine the appropriate measures of response. Heretofore, there has been no portable unit which can perform rapid in situ stand-off, non-contact detection and identification of chemical or biological agents on natural or man-made surfaces.
Typical response protocols of the prior art require that samples be collected (for example in a Biological Integrated Detection System (BIDS) sensor truck, as would be used by military personnel) and then analyzed. This requires emergency response personnel to follow the worst case protocols until the composition of the substance is identified. These procedures can be complex, time consuming, and hazardous, depending on the contaminant. Several techniques involving ion mobility (CAM sensors), surface acoustic waves, and optical detection via fiber optics are currently available for in situ, real time analysis. However, all of these methods require that some part of the instrument come in contact with the sample. This requires emergency personnel to come extremely close to the potentially hazardous substance which is being interrogated. In addition to placing a worker at risk due to his proximity to the unknown contaminant, any time an instrument comes in contact with the contaminant, it must either be disposed of in a controlled manner or thoroughly decontaminated. Both processes are expensive, time consuming and often require elaborate procedures.
Remote sensing of many common airborne chemical species by use of lidar (an acronym for light detection and ranging) devices has been in use since the late 1960""s for atmospheric research and monitoring. However, these lidar systems are intended for long range detection, (i.e., hundreds of meters to kilometers) and are restricted to probing airborne chemicals. Furthermore, these units are often extremely large in size and are not suitable for use in confined spaces such as a subway system or interior spaces of residential or commercial buildings. Several techniques have been employed for determining the composition of the airborne chemical species using spectrally-dependent optical properties. One such method which has been found to be especially useful is Raman spectroscopy.
The idea of Raman stand-off detection is based on the features of Raman scattering. Raman scattering is a two-photon process that conveys information about the vibrational mode-structure of the scattering molecule. In normal Raman scattering, an incident photon of frequency xcexd excites a molecule from its ground electronic level to a xe2x80x9cvirtualxe2x80x9d energy level. If the energy of this virtual level is sufficiently different from that of the nearest real level, the molecule returns quickly back to its ground level; a second photon is emitted almost instantly. If the emitted photon has the same frequency as the incident one, the process is called Rayleigh scattering. However, interaction of the incident photons with the vibrations of a molecule can shift the frequencies of the scattered photons. The shifts are equal to the frequencies of the discrete vibrational modes of the molecule. This unique set of frequency-shifts produces a spectrum that is a vibrational fingerprint of the interacting molecule.
Raman spectroscopy and infrared (IR) absorption spectroscopy are the most common types of vibrational spectroscopy and are complementary to each other. But Raman line positions and relative intensities, (i.e. Raman fingerprints) tend to be nearly independent of the physical state and/or the surrounding environment of the chemical of interest. For example, Raman spectra of substances in water solutions exhibit the characteristic Raman fingerprints of both the substance and the water. For IR spectroscopy, the absorption of the water can be strong enough to completely obscure the IR spectrum of the substance, severely compromising chemical identification. The Raman fingerprint is also independent of the excitation wavelength, a feature unique to Raman spectroscopy that allows the use of any laser excitation wavelength. Hence, Raman detection can be performed in the ultraviolet (UV) solar-blind region of the spectrum (xcex less than 300 nm, where stratospheric ozone absorption attenuates the solar background). Hence, detection can be done during the day, as well as at night, without the presence of a large background signal due to ambient light.
In addition to the advantages of solar-blind UV, the use of a UV source has several more distinct advantages. First, there is the xcexd4-dependence of the Raman scattering intensity on excitation frequency. The system sensitivity will increase by a factor of sixteen whenever the excitation frequency is doubled.
Second, there is a potential for improvement in the scattering cross-section through the phenomenon of pre-resonance or resonance-enhanced Raman scattering. As the excitation photon energy approaches that of an allowed electronic transition (as occurs in the UV for many chemical and biological species), an increase in the Raman scattering cross-section beyond the xcexd4-dependence is observed. This enhancement can approach several orders of magnitude. The combination of the xcexd4-dependence and the potential of pre-resonance or resonance indicates the overwhelming advantage of using UV excitation.
Third, the availability of low-noise optical multichannel analyzers in the visible and ultraviolet can permit capture of a large portion of the Raman spectrum. There is no need to scan the grating of the spectrometer to take a Raman fingerprint, eliminating the need for moving parts in the final instrument design.
Finally, the unique features of Raman spectroscopy make it a viable technique for detection of liquid or solid contamination on the ground or on man-made surfaces. Raman scattering can yield more information from a molecule on a surface than can absorption or fluorescence. In addition, Raman spectra can also yield quantitative information based on the intensity of the Raman signals.
Raman detection also offers the unique possibility of in situ discrimination of chemical and biological contamination. Experimental results suggest that Raman spectroscopy can detect and differentiate biological agents, even with regard to the phase of the substance in its lifecycle. This dual use of the UV Raman stand-off technique means that it could be used as a screening method for the selection of agent-specific, and thus more sensitive detectors.
Various devices have been disclosed which employ Raman spectroscopy to determine the constituents of substances. One such device is set forth in U.S. Pat. No. 5,257,085 to Ulich, et al., (xe2x80x9cUlichxe2x80x9d). Ulich discloses an imaging UV/visible fluorosensing and Raman lidar system comprising an optical sensor for simultaneously measuring temporally, spatially and spectrally resolved laser backscatter from on land, on or beneath the surface bodies of waters and the atmosphere. The lidar system utilizes active or passive interrogation for remote and nondestructive probing of the spectrally-dependent optical properties of a scene. The Ulich device, however, is not a portable compact device which is suitable for use by first responders. The device requires a multitude of reflectors and prismatic elements which would be subject to alignment problems if used in a rugged environment. In addition, the laser beam and optical path of the telescope are not coaxial. Therefore, the laser beam is axially offset from the sight path of the telescope thereby reducing the ability to efficiently package the device.
U.S. Pat. No. 4,945,249 to Grant et al., (xe2x80x9cGrantxe2x80x9d), discloses an apparatus for detecting an anomaly at or near the surface of water or land. The apparatus includes a laser for generating a beam which is sufficiently intense that it causes the anomaly to emit secondary light radiation. The Grant device includes an intensified optical multi-channel detector, which is software configurable and capable of multi-element digitizing. The disclosed system is designed to be airborne and is not compact or portable, as would be required for use by emergency personnel. The system is not intended for short-range (i.e. meters to tens of meters) stand-off distances; consequently, the receiver telescope focus is unable to accommodate such changes in the object distance. Therefore, the device of Grant would not be suitable for in situ interrogation where the environment can present significant spatial constraints. In addition, the device is specifically designed to look for a known substance, i.e. fluorescence from oil seepage and employs a wavelength range, 300-800 nm, which is not in the solar blind region of the spectrum. This makes the device highly susceptible to errors caused by ambient lighting conditions.
Accordingly, it is desirable to provide a portable stand-off sensor which can perform remote, in situ, stand-off analysis of chemical/biological components and is configured such that it can be easily handled by emergency workers without the sensor being detrimentally affected. It would further be desirable to provide a method for remote, stand-off and highly efficient spectroscopic detection of solids, liquids, and gases.
The present invention provides a portable sensing apparatus for the remote interrogation of chemical or biological agents.
The present invention provides a sensing apparatus which permits standoff interrogation of a surface and is configured in a portable package such that the apparatus may be used in physically confined areas.
The present invention further provides an apparatus which includes an optical beam transmitter which transmits a beam comprising at least a pulsed laser emission having an axis of transmission. An optical detector is provided which gathers optical information and has an optical detection path to a target. The path has an axis of optical detection and the transmitter is disposed proximally to the optical detector. A beam alignment device is further provided which fixes the detector proximal to the transmitter and directs the beam to the target along the path such that the axis of transmission is within the path. Operatively connected to the detector is an analyzer for receipt of optical information and analysis of same.
The beam alignment device of the present invention may include a first beam-directing element for altering the axis of transmission of the beam. A second beam-directing element for altering the transmission axis of the beam may be further provided wherein the first beam-directing element directs the beam emitted from the laser at an angle such that the beam crosses into the optical direction path, and the second beam-directing element redirects the beam toward the target. The beam alignment device may also include a rigid structural element to fix the transmitter proximal to the detector.
The analyzer includes a spectrometer for creating a spectra by resolving scattered light energy by wavelength and a transducer for converting the spectra into electrical signals by way of a charged couple device.
In a preferred embodiment, the optical beam transmitter is a pulsed Nd:YAG laser having a wavelength in the solar blind region of the spectrum. Therefore, the sensing apparatus is not affected by ambient light conditions.
The present invention also includes a method for remote, stand-off, high speed and high efficiency spectroscopic detection of solids, liquids and gases, including the steps of:
establishing a co-linear optical transmission/detection path between, a combination optical beam transmitter and spectral analyzer and a target;
transmitting an optical detection beam comprising at least a pulsed laser transmission along the path to illuminate the target;
detecting optical behavior resulting from illumination of the target with the optical detection beam; and
analyzing the optical behavior whereby characteristics of the target are detected.
A preferred form of the sensor, as well as other embodiments, features and advantages of this invention, will be apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.